Apparatus for manufacturing articles by pressing and sintering

ABSTRACT

The invention pertains to the manufacture of articles of thermosetting plastics and other materials. In a first stage, a solid article is formed from a particulate material by introducing such material into a first chamber defined partially by the backing surface of a backing member and partially by the opposed pressing surface of a pressing member. The backing member is frangible and is removably mounted on a support. The first chamber is then closed and the pressing member advanced toward the backing surface to contract the first chamber and compress the particulate material between the pressing and backing surfaces to form the solid article. The pressing member is then retracted, the first chamber is opened, and the backing member and solid article are removed from the support as a unit. Finally, the backing member is fractured and the fragments are removed from the solid article. In a second stage, the convoluted article, or any other similar body is supported in a chamber structure. A filler is disposed between the body and the chamber structure in contact with at least a major portion of the exposed surfaces of the body. The filler and body are then heated, while restricting movement of the filler.

This is a division of application Ser. No. 951,717, filed Oct. 16, 1978,now U.S. Pat. No. 4,252,760.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the manufacture of solid articles fromparticulate materials such as powdered tetrafluoroethylenes and similarthermosetting plastics, as well as ceramics, metals and other materials.The individual particles of such powdered plastics are characterized bynumerous surface irregularities. Thus, when a compressive force isexerted on a quantity of such a powder, these irregularities are causedto interlock with one another to form a solid article. When thecompressive force is applied by fluid pressure via a diaphragm, such aprocess is known as "isostatic molding." The article formed by such apressing process, while solid and integral, is porous and/or brittle.However, if this preliminary article, known as a "preform," is heated toa suitable temperature, the plastic sets, resulting in a finishedarticle which is neither porous nor brittle but rather extremely strongand substantially homogenous in texture. The same basic techniques mayalso be employed in the formation of articles from particulate metals,ceramics, and other materials.

2. Description of the Prior Art

The basic pressing and sintering technique described above is acceptablefor the manufacture of many relatively simple types of plastic articles.However, problems are encountered where the finished article iscomplicated by a convoluted or otherwise irregular configuration,interconnection between the plastic portion of the article and anotherportion of metal or some other substance, and/or an intended use in anenvironment in which extremely good physical properties are needed.

For example, with certain types of articles having complex convoluted orotherwise irregular configurations, it has heretofore been consideredimpossible or at least economically unfeasible to form the article intosuch convoluted configuration from the original powdered plastic. Theproblems of providing properly shaped removable mold pieces isparticularly acute where, for example, the article is a sleeve or otherhollow body which is further internally convoluted. Accordingly, it hasbeen customary to first form a relatively simple body, such as acylindrical sleeve, by the pressing and sintering methods, and thenphysically deform the finished cylindrical sleeve into the desiredconvoluted configuration. In using such a technique, it is necessary tostretch the finished plastic, and the resulting convoluted article thushas a thinner cross-section through the areas which have been sostretched. While these thin-sectioned areas are therefore the weakestpoints of the finished article, they may be located at precisely thepoints at which the greatest strength is needed, an obviousdisadvantage.

In connection with articles comprised of two interconnected bodies, e.g.one of a thermosetting plastic and one of metal, having incompatiblecoefficients of thermal expansion, problems may occur in the sinteringstage of the manufacturing process. For example, the plastic may tend toexpand more than the metal. Then, if the metal and plastic areinterengaged in such a way that the metal inhibits expansion of theplastic at the interface of the two, the plastic may buckle along anexposed or free surface distal the metal portion of the article.

Still another problem is that of producing articles which requiresuperior physical properties. In the past, this has been done byactively exerting pressure on the preform as it is being sintered.However, the use of this technique, commonly known as "pressurecooking," is relatively expensive and increases the cost of the finishedarticle.

The same or similar problems may be encountered in the manufacture ofarticles from particulate materials other than plastics.

SUMMARY OF THE INVENTION

The present invention provides an apparatus by which a convoluted orotherwise irregularly shaped preform may be formed by compaction of aparticulate material without undue expense or complication. Theinvention further comprises improvements which provide a finishedarticle having superior physical properties without the expense andcomplication of prior "pressure cooking" techniques. Furthermore, withregard to articles comprised of two bodies having incompatiblecoefficients of thermal expansion, the invention eliminates the problemof buckling of one of the bodies along a free or exposed surface.

More specifically, the invention comprises a press apparatus including asupporting assembly, a frangible backing member removably mounted on thesupporting assembly and having a backing surface, and a pressing meansmounted on the supporting assembly and having a pressing surface opposedto the backing surface. The backing and pressing surfaces may beconvoluted or otherwise irregularly configured in accord with thedesired shape of the finished article. The supporting assemblycooperates with the backing member and pressing means to define a firstchamber between the backing surface and the pressing surface. Inoperation, a particulate material such as a powdered thermosettingplastic is introduced into the first chamber. The pressing means is thenadvanced towards the backing surface to contract the first chamber andcompress the particulate material to form a solid preform. The pressingmeans is then retracted, the chamber opened, and the backing member andpreform removed as a unit. The backing member may then be fractured andthe fragments removed from the solid preform either before or aftersintering.

In preferred embodiments, the pressing means is an elastomeric bladder,or series of bladders, which are advanced and retracted by introducingfluid into a second chamber or chambers located across the pressingmeans from the backing member and removing fluid from said secondchamber(s). The frangible backing member is preferably formed, forexample, of plaster-of-Paris or some other relatively inexpensivematerial. Such backing members are inexpensive enough to permit the useof a new backing member for each article formed in the pressingapparatus.

After a preform body has been manufactured, such body is sintered byfirst supporting restraining means in contact with at least a majorportion of the free surfaces of the body, and then heating the body andthe restraining means while restricting movement of the latter. It isnot necessary to actively exert pressure on the free surfaces duringheating, but rather, only to provide sufficient force for holding therestraining means against such surfaces when cold and resist movementwhen heated. As the body expands during the heating step, it exerts asufficient force against the restraining means to substantially improvethe physical properties of the resulting sintered article. Additionally,the use of the restraining means will often cause healing of cracks orother defects in a preform, thus permitting use of preforms which wouldhave to have been discarded with other methods.

Where the article in question is comprised of two interconnected bodiesformed of respective materials having incompatible coefficients ofthermal expansion, the above heating method can be used for theadditional or alternative purpose of preventing buckling of one of thebodies along the free surface thereof contacted by the restrainingmeans.

In any event, the heating is preferably accomplished by supporting thearticle or body in question in a chamber structure and placing a fillerinto the chamber structure to fill the space between the latter and thefree surfaces of the body in question. The filler may be a flowablesubstance poured into this space whereby the filler contacts andconforms to the free surfaces of the body even though they may beintricately convoluted or otherwise irregular. The filler medium may beone, such as plaster-of-Paris, which is inexpensive and which sets toform a solid filler member. Thus, after the heating process, such fillermember can simply be broken away and discarded like the backing memberused in the pressing stage of the formation of a convoluted article asdescribed hereinabove.

In that connection, where the article to be sintered is a convolutedarticle which has been formed in accord with the first stage of thepresent invention, the backing member used in that stage may be left inplace for the heating stage, whereby it is only necessary to pour thefiller medium about some of the surfaces of the article to be sintered.If the backing member is annular and is surrounded by the body to besintered, a mandrel or the like may be provided to fit within thebacking member and prevent it from being crushed by the expandingplastic body. Then, after heating, the backing member, plastic body andfiller member, may be removed from the chamber structure and themandrel, and the backing member and filler member may be broken away anddiscarded.

Accordingly, it is a principal object of the present invention toprovide an improved apparatus for manufacturing solid articles fromparticulate materials.

Another object of the present invention is to provide such an apparatusemploying a frangible backing member.

Still other objects, features and advantages of the present inventionwill be made apparent by the following detailed description of thepreferred embodiments, the drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view through a press apparatusaccording to the present invention.

FIG. 2 is a view similar to that of FIG. 1 showing the press apparatusin a different position.

FIG. 3 is a transverse sectional view taken along the lines 3--3 of FIG.1.

FIG. 4 is a longitudinal sectional view of an apparatus for sinteringthe article formed in the press of FIGS. 1-3.

FIG. 5 is a sectional view of an apparatus for sintering a differenttype of article.

FIG. 6 is an elevational view of the article to be treated in theapparatus of FIG. 5, viewed in the direction indicated by arrows 6--6 inFIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-3 disclose a press apparatus according to the present inventionfor forming a convoluted sleeve from a powdered thermosetting plasticmaterial such as a tetrofluoroethylene. Such sleeves are used in thefabrication of expansion joints. The apparatus comprises a supportingassembly including a base plate 10. The supporting assembly furtherincludes a mandrel 12 secured to base plate 10 by bolts 14 and extendingupwardly from base plate 10. Mandrel 12 has a large diameter lowerportion defining an upwardly facing shoulder 12a and having a bevelledannular flange 12b extending radially outwardly therefrom. Aboveshoulder 12a, mandrel 12 has a tapered portion 12c. Above portion 12c,the diameter of mandrel 12 is further reduced to form another upwardlyfacing shoulder 12d and a central cylindrical portion 12e. Above portion12e, the diameter of mandrel 12 is further reduced to form an upperportion 12f, the upper end of which is externally threaded.

The supporting assembly further comprises a support ring 16 which isemplaced on mandrel 12 so that it rests on shoulder 12a thereof andsurrounds tapered portion 12c. Support ring 16 has a planar uppersurface 16a which lies in a common plane with shoulder 12d of mandrel12.

After the base plate 10, mandrel 12, and ring 16 have been assembled asshown, an annular backing member 18 is emplaced over mandrel 12 insurrounding relation to portion 12e and with its lower end resting onshoulders 12d and 16a. Backing member 18 has a cylindrical inner surface18a sized for a sliding fit on mandrel portion 12e. The radiallyoutwardly directed surface 18b of backing member 18 has a convolutedconfiguration. More specifically, surface 18b, which serves as thebacking surface of member 18, has a major central portion which isgenerally sinusoidal when viewed in longitudinal section as in FIG. 1.Thus, surface 18b includes convex apical segments 18c alternating alongits length with concave apical segments 18d, the adjacent apicalsegments 18c and 18d being interconnected by inclined connectingsegments 18e. Surface 18b also includes short generally cylindricalsegments adjacent its upper and lower edges. Backing member 18 isfrangible, preferably being formed of a relatively inexpensive materialsuch as plaster-of-Paris.

After emplacement of backing member 18, additional parts of thesupporting assembly are emplaced. Specifically, a pair of semi-annularrigid side members 20 and 22 are positioned to form a complete annulusaround mandrel 12 (compare FIGS. 1 and 3). Side members 20 and 22 haverespective grooves 20a and 22a configured to mate with flange 12b ofmandrel 12 to properly position the side pieces 20 and 22 on themandrel. The outer surfaces 20b and 22b of side pieces 20 and 22respectively are inclined inwardly and upwardly. Thus, pieces 20 and 22may be held in place by a retaining ring 32 having a correspondinglytapered frustoconical inner surface 32a. When ring 32 is emplaced insurrounding relation to side members 20 and 22, the engagement ofsurface 32a with surfaces 20b and 22b serves to hold member 20 and 22together with a wedging action while also permitting ring 32 to besupported on members 20 and 22. Alternatively, the side members 20 and22 may be temporarily held in place by rams or the like acting radiallyinwardly on the members, or in some cases, by their own weight.

Each of the side members 20 and 22 has, on its radially inner side, asemi-annular recess 20c or 22c respectively. Recesses 20c and 22c carryrespective annular bladders 24 and 26 of polyurethane or anothersuitable elastomer. Bladders 24 and 26 are hollow and encase respectivecore members 28 and 30. Viewed in longitudinal cross-section, each ofthe bladders 24 and 26 comprises a relatively thick radially outerportion which is generally U-shaped and fits within the respectiverecess 20c or 22c. Each bladder further comprises a relatively thinradially inner portion which protrudes from the respective recess 20c or22c. The surfaces 24a and 26a respectively of these inner bladderportions face generally radially inwardly and oppose the backing surface18b of backing member 18. Surfaces 24a and 26a serve as the pressingsurfaces of the apparatus.

Referring still to FIG. 1, it can be seen that the relatively thinradially inner portions of the bladders 24 and 26, as well as thepressing surfaces 24a and 26a which they define, generally follow thesinusoidal configuration of surface 18b. For example, surface 24aincludes concave apical segments 24b each opposed to a respective one ofthe convex apical segments 18c of backing surface 18b. Alternating withthe concave apical segments 24b are convex apical segments 24c eachopposed to a respective one of the concave apical segments 18d of thebacking surface. Joining adjacent ones of the apical segments 24b and24c are connecting segments 24d, each inclined to generally parallel arespective opposed one of the connecting segments 18e of the backingsurface. In addition, surface 24 has end segments 24e and 24f facingupwardly and downwardly respectively. These segments 24e and 24f adjointhe relatively thick base portion of bladder 24 but are spacedlongitudinally inwardly from the upper and lower extremities of saidbase portion for a purpose to be described more fully below. Therelatively thin radially inner portion of bladder 26 is the mirror imageof the corresponding portion of bladder 24, and in particular, defines apressing surface 26a having alternating convex and concave apicalsegments interconnected by connecting segments, as well as upwardly anddownwardly facing end segments, as shown in FIG. 1.

The rigid cores 28 and 30 encased in respective bladders 24 and 26 haveexternal surfaces conforming to the configuration of the internalsurfaces of said bladders. Cores 28 and 30 are not bonded or attached tothe respective bladders, although in alternate embodiments they could beso attached along the relatively thick outer portions of the bladders.However, the cores do contact the entire inner bladder surfaces when thebladders are in their relaxed positions as shown in FIG. 1. Thus, cores28 and 30 serve to retain the relatively thick base portions of bladders24 and 26 in their respective recesses 20c and 22c as well as to form aconvoluted support surface for the relatively thin inner bladderportions. Cores 28 and 30 may thus be considered part of the supportingassembly of the apparatus.

It can be seen that a generally annular convoluted first chamber isdefined between the backing surface 18b of backing member 18 and thepressing surfaces 24a and 26a of bladders 24 and 26. A pair of generallysemi-annular second chambers may be said to be formed within thebladders 24 and 26 respectively and particularly between the thinconvoluted inner portions thereof and the adjacent core surfaces. Withthe bladders in their relaxed positions, as shown in FIG. 1, the secondchambers are of negligible volume due to the abutment of the bladderswith their respective cores 28 and 30. However, it can be seen that, byintroducing fluid to the interfaces between the convoluted portion ofeach bladder and its core, the relatively thin inner or convolutedportions of the bladders may be advanced toward the backing surface 18bof the backing member, and by withdrawing fluid from the interiors ofthe bladders, the relatively thin convoluted bladder portions, may beretracted away from backing surface 18b. Thus, the relatively thinconvoluted bladder portions serve as pressing members which may beadvanced toward and retracted from backing surface 18b to respectivelycontract and expand the first chamber formed between backing surfaces18b and pressing surfaces 24a and 26a. These pressing members aresufficiently elastomeric to prevent such movement, but do retain theirbasic convoluted configuration.

In order to provide for such introducing of fluid to the interiors ofthe bladders as well as the withdrawal of such fluid, passageways areprovided through the support assembly. In particular, the relativelythick base portion of bladder 24 has a radial bore 24g therethroughaligned with bores 34 and 36 through core 28 and side member 20respectively. A nipple 38 is mounted in bore 24 and extends intoenlarged portions of bores 34 and 36 to connect the latter two bores andthe members in which they are formed. Bore 36 has its radially outer endclosed by a plug 40. Another bore 42 intersects bore 36 intermediate itsends and extends downwardly therefrom through side member 20 and has itslower end closed by a plug 44. Bore 42 is intersected just above itslower end by a short radial bore 46 extending outwardly through sidemember 20 and having its outer end enlarged and threaded as indicated at46a for connection to a fitting at the end of a hydraulic line. Whensuch a line is connected at 46a, hydraulic fluid can be admitted to thechamber within bladder 24 via bores 46, 42 and 36, nipple 38, and bore34. Such fluid can likewise be removed from the chamber formed withinbladder 24 via the same passageway system. A similar passageway system(not shown) is provided for introducing hydraulic fluid into the chamberformed within bladder 26 and removing such fluid.

When the parts of the apparatus heretofore described have beenassembled, a particulate material, specifically a powdered thermosettingplastic 56 such as a tetrofluoroethylene, is introduced into the firstchamber defined by surfaces 18b, 24a, 26a and the upper surface of ring16. The chamber is filled with such powder. The apparatus is vibrated ortapped with a suitable tool to cause the powder to settle into allportions of the chamber and eliminate air pockets. Additional powder maybe added as needed to completely fill the chamber.

Assuming ring 32 has already been emplaced, the chamber may then beclosed by a ring 48 having a sliding fit on the unthreaded part of upperportion 12f of mandrel 12. The abutting radial surfaces of ring 48 andside members 20 and 22 are downwardly and inwardly tapered as shown sothat, as ring 48 is moved downwardly on mandrel 12, it will be stoppedin a position such that its lower surface is flush with the upper endwalls of recesses 20c and 22c. As mentioned hereinabove, the endsegments, e.g. 24e and 24f of the pressing surfaces 24a and 26a arespaced longitudinally inwardly from the longitudinal extremities of thethick base portions of the bladders which fill recesses 20c and 22c, andthus from the upper and lower surfaces of rings 16 and 48, respectively.Thus, these end segments define, with rings 16 and 48, radiallyextending portions of the first chamber for receipt and formation of endflanges on the plastic article to be formed. After ring 48 has beenproperly emplaced, an annular cover 50 having a central bore for receiptof upper mandrel portion 12f may be emplaced. Cover 50 is secured toretaining ring 32 by screws, one of which is shown at 52, and to ring 48by other screws, one of which is shown at 54. Additional screws, one ofwhich is shown at 55, are threaded into cover 50 to abut the uppersurface of ring 32. Thus, by manipulating screws 55 and 52, it ispossible to adjust the relative positioning of members 32, 48 and 50,which together form a closure assembly. Specifically, the tightness withwhich such assembly will engage side members 20 and 22 may be adjusted.

As mentioned above, the ring 32, in many instances, may not be placed onmembers 20 and 22 prior to filling of the first chamber with powder 56.In such instances the closure assembly 32, 48, 50 may be pre-assembledand placed on the members 20, 22 as a unit. Or ring 32 may be pre-placedand members 48 and 50 pre-assembled and mounted as a unit. In any event,a large nut 59 is threaded onto the upper end of mandrel portion 12f toretain the closure assembly 32, 48, 50 in place.

With the apparatus thus completely assembled, hydraulic fluid isintroduced into the second chamber defined within bladders 24 and 26 toadvance the thin inner portions or pressing members thereof as shown inFIG. 2. This contracts the first chamber formed between surfaces 18b,24a and 26a, and rings 16 and 48 to compact the powdered plastic 56therein. The surface irregularities in the particles are thus forcedinto interlocking engagement so that the individual particles are joinedto form a solid article, in this case a plastic preform 56' for anexpansion joint. Such article has radially extending end flanges 56a and56b and a convoluted generally sinusoidal central portion, allcorresponding to the general configuration of said first chamber. Morespecifically, the central portion of the preform has apical segments56b, alternately inwardly and outwardly concave, and interconnected byinclined connecting segments 56c.

As mentioned hereinabove, pressing surfaces 24a and 26a generally followthe configuration of backing surface 18d. However, these surfaces arenot truly parallel. Rather, the backing member 18 and the bladders 24and 26 (more particularly the pressing members formed by the relativelythin inner portions thereof) are configured so that the backing andpressing surfaces are spaced apart by a greater distance at therespective opposed apical segments thereof than at the respectiveopposed connecting segments thereof. Accordingly, the preform producedby the apparatus is thicker at its apical segments 56b than at itsconnecting segments 56c. This is desirable since, in an expansion joint,the greatest strength is needed at the apical segments. Thus, theexpansion joint produced by the apparatus and method of the inventionincorporates a superior quality impossible to obtain with prior arttechniques in which a cylindrical sleeve is formed from the powerderedplastic, completely sintered, and then stretched or deformed into thedesired convoluted configuration.

After formation of preform 56', fluid is withdrawn from the secondchamber within bladders 24 and 26 to retract the pressing members formedby the thin inner portions of said bladders. Nut 59 is then removed frommandrel 12, and closure assembly 32, 48, 50 is removed from side members20 and 22. Next, the side members themselves are removed from the base10 and mandrel 12. Removal of the side members is facilitated by theprovision of a flange 60 extending laterally outwardly from member 20for grasping said member and removing it. Member 22 would have a similarflange (not shown). Preform 56' and backing member 18 are next removed,as a unit, from mandrel 12 by sliding them upwardly therealong. Tofacilitate such removal, a plurality of pushrods 62 are longitudinallyslidably mounted in base 10 and the enlarged lower portion of mandrel 12to abut the ring 16. Thus, by pushing upwardly on rods 62, backingmember 18 and preform 56' may be urged upwardly, with ring 16 serving asa bearing member.

The preform 56' thus produced is sufficiently solid to hold the desiredshape in the absence of any substantial loading, but is neverthelessrelatively porous and brittle. In order to set the plastic and form astrong homogenous article, preform 56' must be sintered or heated. FIG.4 shows an apparatus for sintering the preform 56'. This apparatusincludes a bowl-shaped chamber structure 64 having cylindrical sidewalls and a generally planar bottom wall. The bottom wall has aninternal recess 64a for receipt of the lower end of a mandrel 66. Thebottom wall of chamber structure 64 is bored and the lower end ofmandrel 66 is provided with a threaded bore whereby they may beremovably secured together by a screw 68. The outer diameter of mandrel66 is equal to that of the portion 12e of the mandrel 12 of the presspreviously described. Thus, backing member 18 may be slidably mounted onmandrel 66 along with the surrounding preform 56' as shown, with thelower ends of member 18 and preform 56' resting on the bottom wall ofchamber structure 64. Thus, the lower and radially inner surfaces ofpreform 56' are in abutment with the bottom wall of chamber structure 64and with backing member 18 respectively. However, the radially outwardlyfacing surface and the upper surface of preform 56' are left free orexposed. In accord with the present invention, a filler is introducedinto chamber structure 64 so that it contacts these free surfaces andfills the space between preform 56' and the chamber structure.Preferably, the filler is a flowable substance which can be poured intochamber structure 64 and will flow into and around the convolutions ofpreform 56', conforming thereto. In the embodiment shown, the fillermedium is plaster-of-Paris 68 which may be poured into chamber structure64 via a fitting (not shown) for a threaded radial bore 70 near thelower end of the side walls of said structure and/or through the openupper end of structure 64. Bore 70 may then be closed by a plug 72.Chamber structure 64 may then be vibrated or tapped lightly with asuitable tool to release any air which may be trapped in the plaster andto cause the plaster to settle into the convolutions of preform 56'.More plaster may then be added through the upper end of chamberstructure 64 as needed. It will be noted that both chamber structure 64and mandrel 66 extend upwardly substantially beyond the upper end ofpreform 56' whereby the plaster 68 may be poured over the free uppersurface of the preform.

Plaster 68 is allowed to set to form a solid filler member. The chamberis then closed by a generally disc-shaped cover 74 sized to slidably fitwithin chamber structure 64 and having a central recess 74a for receiptof the upper end of mandrel 66. Cover 74 and mandrel 66 are threaded toreceive a screw 76 for retaining the cover in place.

The entire assembly pictured in FIG. 4 is then placed in a suitable ovenand sintered or heated. During the heating process, preform 56' willtend to expand. Although the plaster members 18 and 68 and thesurrounding metal chamber structure 64 may also expand slightly, chamberbowl 64 and mandrel 66 will generally restrict movement of the plastermembers radially, i.e. away from each other. Accordingly, members 18 and68 will serve as restraining means providing a reaction to the expansiveforces of preform 56', and this force will be substantially evenlydistributed over all the surfaces of the preform. This reaction forcesubstantially improves the physical properties of the finished articleproduced from preform 56', and can even serve to "heal" cracks or otherdefects in the preform. Nevertheless, it will be appreciated that theseeffects are achieved without the need for the active exersion of highexternal pressures on the preform, and thus without complicated andexpensive equipment. On the contrary, a relatively simple chamberstructure is utilized which merely provides sufficient support to resistmovement of the plaster members, and the expanding preform itselfprovides the necessary force.

After heating, the entire apparatus of FIG. 4 is cooled. Screw 76 isthen removed, as is cover 74. Next, mandrel 66 may be released byremoval of screw 68, and then lifted upwardly and out of the surroundingplaster member 18. If needed, push rods may be provided in the lowerwall of chamber bowl 64 to facilitate removal of the mandrel. Finally,the finished convoluted joint formed from preform 56' is removed fromthe chamber bowl along with the engaged plaster members 18 and 68.Because the latter members are frangible and relatively inexpensive,they can then be removed from the convoluted joint by simply fracturingthem, removing the fragments thereof, and discarding them. Theplaster-of-Paris is sufficiently inexpensive so that new plaster membersmay be employed for each joint which is produced. The frangibility ofthese members allows a relatively complicated convoluted member to beformed without the need for expensive breakdown mold pieces, orsacrificing of the optimum configurational and physical properties ofthe finished article. It can also be appreciated that, even if preform56' is to be sintered according to a method other than that prescribedabove, backing member 18 can be fractured and removed from the preformprior to sintering. For example, in some processes, it may be desirableto so remove the backing member and pour wet plaster into the interiorof the preform prior to sintering in substantially the same manner asplaster 68 is poured around the exterior of the preform. Also, in sometypes of articles in which the optimum physical properties achieved bythe aforementioned sintering methods are not necessary, the preform maybe sintered in a more conventional free-sintered mode.

The sintering method of the present invention has even furtheradvantages when adapted for the heating of articles comprised of twointerengaged bodies having incompatible coefficients of thermalexpansion. An example of such an article is the combination valve stemand disc for a butterfly valve shown in FIGS. 5 and 6. This articlecomprises an inner metal body which includes a central disc-shaped core80 with integral valve stem portions 82 and 84 extending therefrom. Atthe juncture of each of the valve stem portions 82 and 84 with the discportion 80, there is a respective annular groove 86 or 88. A secondgenerally disc-shaped body 90 of a thermosetting plastic surrounds thecentral portion of the metal body including the disc portion 80, thegrooves 86 and 88, and the adjacent inner extremities of the valve stemportions 82 and 84. Body 90 is formed around the metal body 80, 82, 84by a pressing process, preferably an isostatic molding process, so thatit exists in a preform state which must be sintered. The plastic preformbody 90 extends into and fills the grooves 86 and 88, forming therewithinterlock formations. It can be seen that if this article were sinteredin a free-centered condition, i.e. with the outer surfaces of body 90exposed or free, the metal body would resist inward expansion of body90, while the interlocking of body 90 with grooves 86 and 88 wouldresist longitudinal expansion. Accordingly, the body 90 could onlyexpand laterally outwardly and might tend to buckle along its outersurfaces.

To resolve this problem, while simultaneously producing an article inwhich the plastic portion has superior physical properties, the articleis sintered in an apparatus such as that shown in FIGS. 5 and 6 inaccord with the present invention. Said apparatus comprises a pair ofgenerally cylindrical chamber members 92 and 94. When placed inend-to-end abutment, members 92 and 94 define a large cylindricalchamber having an inner diameter substantially equal to that of thedisc-shaped body 90. Body 90 and the enclosed portions of the metalbody, 80, 82, 84, are positioned within the cylinder defined by members92 and 94, the abutting surfaces of said members being recessed asindicated at 96, 98, 100 and 102 to accommodate the parts of valve stemportions 82 and 84 which extend outwardly from plastic body 90. Chambermembers 92 and 94 may then be secured together by an annular clamp 104which engages annular flange 106 and 108 on members 92 and 94respectively. With the valve element thus mounted in the apparatus, thebody 90 thereof has two free or exposed surfaces 90a and 90b generallydefining the opposite sides of the valve disc. Fillers are thenintroduced into the chamber defined by members 92 and 94 in contact withsurfaces 90a and 90b. While such fillers may be flowable media, as inthe embodiment of FIG. 4, the relatively uncomplicated configuration ofsurfaces 90a and 90b makes it more feasible to use prefabricated solidfiller members. In any event, the filler media or members are preferablyformed of an inexpensive frangible material such as plaster-of-Paris.

A first filler member 110 may be provided either by pouring wet plasterthrough the adjacent open end of the cylindrical chamber structure, i.e.the end defined by member 94, or by inserting a prefabricated plastermember, formed to match the configuration of surface 90a, through saidopen end. If the wet plaster is used, it is then allowed to set to forma solid filler member 110. Member 110 is then fixed in place by a cover112 having a plug portion 112a sized to fit within the chamber structureand a flange portion 112b disposed to overlie the outer end of chambermember 94. Plug portion 112a is advanced into chamber member 94 until itabuts filler member 110, and then flange portion 112b is secured tochamber member 94 by screws 114 to retain cover 112 in that position sothat it will resist movement of filler member 110 away from the metalbody 80, 82, 84. The apparatus is then inverted and a similar insertmember 116 is installed in chamber member 92 in contact with freesurface 90b of body 90 in substantially the same manner as with fillermember 110. Filler member 116 is then retained in place by a cover 118,identical to cover 112, secured to chamber body 92 by screw 120.

The entire assembly pictured in FIG. 5 is then placed in an oven forsintering of body 90. As the assembly is heated, body 90 will tend toexpand. As mentioned above, the metal body encased within plastic body90 inhibits expansion of the latter in any direction other thanlaterally outwardly. However, as body 90 tends to expand laterallyoutwardly the filler members 110, and 116 abutting surfaces 90a and 90bprovide a resistance force. While the filler members 110 and 116 and thechamber structure and covers may expand slightly, the covers 112 and 118offer sufficient resistance to movement of filler members 110 and 116away from metal body 80, 82, 84 to permit the filler members to providea substantial resistance force along surfaces 90a and 90b of plasticbody 90. Because the filler members abut substantially the entirety ofthese surfaces, this resistance force prevents buckling of the plasticbody along surfaces 90a and 90b. In addition, the use of the fillermembers provides all the advantages described in connection with theembodiment of FIG. 4, in particular, improved physical properties of theresulting plastic article and healing of cracks or other defects in thepreform.

After the apparatus has been cooled, screws 114 and 120 are removed,covers 112 and 118 are removed, clamp 104 is disengaged, and the chambermembers 92 and 94 are separated. It may be possible to simply lift thefiller members 110 and 116 from the finished plastic body 90. However,if necessary, members 110 and 116 may be fractured for removal and thefragments discarded due to the frangibility and inexpensive nature ofthe members.

While the foregoing represents preferred embodiments of the methods andapparatus of the present invention, numerous variations of those methodsand the apparatus are encompassed within the spirit of the invention.For example, in the press apparatus illustrated in FIGS. 1-3, twoseparate semi-annular bladders are employed to define the pressingmeans. This allows for easy disassembly of the apparatus to permitremoval of the finished preform. However, if the cores 28 and 30 wereomitted, and the bladder material sufficiently flexible, a singleannular bladder could be employed. Various other changes in theconfiguration of the various parts of the apparatus could also be made,and in particular, the apparatus could be adapted to mold articleshaving different configurations from the convoluted sleeve illustrated.The backing member 18, while disclosed as being comprised of plaster,could also be comprised of other frangible materials. The inexpensivenature of the backing member makes it possible to use a new backingmember for each article molded, merely fracturing and discarding thebacking member after the molding process is completed. The backingsurface of the backing member can be configured to form various types ofconvolted or otherwise irregular, i.e. non-planar, surfaces which, dueto their shape and/or position on the article being formed, would bedifficult to mold with conventional isostatic apparatus.

In connection with the heating portions of the process illustrated inFIGS. 4 and 5 it is possible to use either prefabricated solid fillermembers or flowable filler media, as mentioned above. Whileplaster-of-Paris may be advantageously used in either the flowable orsolid form, numerous other materials may be used. For example, ifprefabricated filler members are to be employed, they may be formed of asuitable metal. If a flowable medium is employed, it may comprise aliquid, a semi-liquid, or even a granular material such as sand, as longas it is sufficiently flowable to conform to the configuration of thefree surface or surfaces of the body to be heated. Again, the medium ispreferably an inexpensive one which can be discarded after each heatingprocess. Various changes can also be made in the apparatus used toretain the filler media or members in place, and in particular, suchmeans may be varied in accord with the configuration of the articlebeing treated.

Numerous other modifications will suggest themselves to those of skillin the art. Accordingly, it is intended that the scope of the inventionbe limited only by the claims which follow.

We claim:
 1. A press apparatus for forming a solid article from aparticulate material comprising:a supporting assembly; a frangiblebacking member removably mounted on said supporting assembly and havinga backing surface; a pressing means mounted on said supporting assemblyand having a pressing surface opposed to said backing surface, saidsupporting assembly being cooperative with said backing member and saidpressing means to define a first chamber between said backing surfaceand said pressing surface for receipt of said particulate material, andsaid pressing means being advanceable and retractable toward and awayfrom said backing surface to contract and expand said first chamber;closure means for selectively opening and closing said first chamber;and means for selectively advancing and retracting said pressing means.2. The apparatus of claim 1 wherein said backing member is comprised ofplaster.
 3. The apparatus of claim 1 wherein said supporting assemblyfurther defines with said pressing means second chamber means locatedacross said pressing means from said backing member and said means foradvancing and retracting said pressing means comprises means forselectively introducing fluid into and removing fluid from said secondchamber means.
 4. The apparatus of claim 3 wherein said pressing meanscomprises elastomeric diaphragm means.
 5. The apparatus of claim 4wherein said backing and pressing surfaces are generally annular andcoaxial.
 6. The apparatus of claim 5 wherein said supporting assemblyincludes a mandrel and said backing member is generally annular andslideably mounted on said mandrel, said backing surface facing generallyradially outwardly therefrom, and said pressing means surrounding saidbacking member.
 7. The apparatus of claim 6 wherein said backing andpressing surfaces are longitudinally convoluted, including respectivealternating apical segments and connecting segments, said backing andpressing surfaces being generally parallel but spaced apart by a greaterdistance between respective opposed ones of said apical segments thanbetween respective opposed ones of said connecting segments.